Place chicken breasts between two pieces of wax paper, and pound to 1/4 inch thick with a meat mallet. Dust chicken with flour, salt , pepper and oregano.In a skillet, fry chicken in olive oil over medium heat. Cook until done, turning to cook evenly. Set aside, and keep warm.In the same pan, melt the butter over medium heat; add mushrooms, sun-dried tomatoes, and Marsala wine. Cook for approximately 10 minutes, stirring occasionally. Mix in spinach, and cook for about 2 minutes. Serve over chicken.